High-frequency transmission system



Oct. 18, 1949. w. E. BRADLEY 2,435,031

HIGH-FREQUENCY TRANSMISSION SYSTEM Original Filed Aug. 30, 1944 2 Sheets-Sheet l TO "*LOAD TO OSCILLAT 4] I LO TO "l g LATOR M F IG 2 INVENTOR.

WILLIAM E. BRADLEY ATTORNEY Oct. 18, 1949. w. E. BRADLEY HIGH-FREQUENCY TRANSMISSION SYSTEM 2 Sheets-Sheet 2 Original Filed Aug. 30, 1944 OSCILLATOR FIG. 4.

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WILLIAM EfiBRADI EY BY ONM F J) ATTORNEY Patented Oct. 18, 1949 HIGH-FREQUENCY TRANSMISSION SYSTEM 'William E. Bradley, Swarthmore, Pa., assignor, by mesne assignments, to Philco Corporation, Philadelphia, Pa., a corporation of Pennsylvania Original application August 30, 1944, Serial No. 551,951. Divided and this application August 31, 1944, Serial No. 552,176

11 Claims.

This application, which is a division of application Serial No. 551,951, filed August 30, 1944, relates to a frequency stabilizing system and more particularly relates to a system for adjusting and stabilizing the frequency of high frequency oscillators as thevoltage, current or load impedance varies.

In accordance with my invention, frequency stabilization is effected by reflecting a reactance into the line in response to a shift in frequency from normal of a kind that will tend to restore the frequency'to its original value.

More specifically, in one form of my invention I connect a parallel reactance resonant to the frequency to be controlled across the transmission line. There are a number of points along the transmission line one half wave length apart, where an inductive reactance applied to the transmission circuit will elevate and a capacitative reactance will lower the frequency of the oscillator. At one of these points where this effect is maximum, I shunt my parallel resonant circuit across the transmission line which matches the impedance of the load.

If now the load impedance so changes that the frequency rises, a capacitance is presented by the parallel resonant circuit to the transmission line which lowers the frequency to its normal value.

If the load impedance so change-s that the frequency falls, inductance is presented to the transmission line which raises the frequency to its normal value.

Halfway between the points along the transmission line, referred to above, an inductive reactance will lower and a capacitative reactance will elevate the frequency. This reactance is secured by a resonant series inductance and capacitance connected in series in the system at one of these half way points.

If the frequency rises in response to a change in load impedance above the stabilizing value and therefore above resonance frequency, an inductance is presented by the series resonant circuit to the transmission line which lowers the frequency to its normal value.

If the load impedance so changes that the frequency falls, capacitance is presented to the transmission line which raises the frequency to its normal value.

In the discussion above, I have referred to a special network. This, as will be clear from the description to follow, can also be a network in the coaxial sense, i. e., an arrangement of coaxial transmission line or it may be an arrangement of wave guide sections or resonant cavities. Ar-

rangements can also be made using combinations of network elements, coaxial lines, Wave guides and resonant cavities as will be explained hereinafter.

Accordingly, an object of my invention is to provide a novel network arrangement for effecting frequency stabilization of a system.

A further object of my invention is to provide a novel reactance network coupled to a circuit, the reactance reflected into the oscillating circuit being a function of the frequency fluctuations from normal to maintain the frequency of the system substantially constant.

Still another object of my invention is to provide a novel resonant parallel inductance capacitancecircuit so connected in a transmission system that it presents capacitance to the line when the frequency rises to lower the frequency to normal and presents inductance to the line when the frequency drops to raise the frequency to normal.

Another object of my invention is to provide a novel resonant series inductance capacitance circuit so connected in a transmission system that it presents inductance to the line when the frequency rises to lower the frequency to normal and presents capacitance to the line when the frequency drops to raise the frequency to normal.

Still a further object of my invention is to provide a novel frequency controlled stabilizer which tends to maintain the load impedance substantially constant.

These and other objects of my invention will appear from a detailed discussion which follows in connection with the drawings, in which Figure 1 shows a method of connecting coaxial lines in parallel.

Figure 2 shows a method of connecting coaxial lines in series.

Figure 3 shows the invention applied to a resonant cavity with acoaxial line.

Figure 4 shows in cross-section a further spec'ific construction of a coaxial line embodiment of my invention.

Figure 5 shows a further development of my invention utilizing the series coaxial line junction illustrated in Fig. 2.

Figure 6 is the schematic equivalent of Figure 3.

Referring now to the drawings, I have shown my invention applied to coaxial systems for micro-wave frequency transmissions. The coaxial line 3|, 32 conducts power from the oscillator to the load. The oscillator such as a magnetron may have two or more modes of oscillations. At such a position on this line in relation to the oscillator that the addition of a capacitance low- 3 ers the frequency a maximum amount, a stub line 33, which is an odd number, of quarter waves long, is inserted at right angles to and in electrical parallel connection with the main line.

One end of stub line 33 is open and fits into an opening in the outer shield of coaxial-line 3|. The inner conductor of the stub extends through the junction of the stub and main coaxial line and is connected to the inner conductor of the main coaxial line. The opposite end of thestub is closed off at 34, so that it presents an infinite impedance to the main line 3|, 32 at the normal frequency. If now the load is changed'so that the frequency tends to decrease, this stub line oper ates as a less than quarter waveiline, andthus operates to add inductance totheline andthereforei to raise and restore the frequency to its normal value.

Because of its position on the line, its action counteracts the change in load impedance. Conversely, if the load impedance so changes that it acts as an inductance at the junction point of the main line and stub andthe frequency-rises, the stub line operates as a greater than quarterwave line and thus adds capacitance to the line causing the frequency to restore itself.

A coaxial series connection is shown in Figure 2. The central conductor 4I passes straight through the system. The outer conductor 42' of the main transmission line formed by M and 42 is broken at 43 at a point where a series inductance introduced into the transmission line decreases the frequency and a capacitance introduced into the line raises the frequency.

This break leads into a section of transmisf sion line with the outer surface of 42 acting as the inner conductor and with a hollow cylindrical tube 44 acting as the outer conductor. The ends 45 and 4tv of the tube are closed and in contact with conductor 42 along the circumferences 41 and 48 respectively.

The length of this section of 46 to 41 is chosen to be one half wave length or any other integral number of halves of a wave length of the frequene cy to be stabilized. This means that the impedance looking into this section of the line from the main transmission line is substantially a short circuit. Consequently, there results an ef-' fective short circuit at 43 when the stabilizing frequency exists.

At the frequency to be stabilized, the stabilizing unit does not have any effect upon a transmission through the main transmission line from the oscillator to the load.

If an impedance of the load is changed so that the impedance looking into the main section of the transmission line at position 43' appears to have a capacitance added to it, the frequency of the oscillator rises.

As the frequency begins to increase, the impedance looking into the compensator no longer appears as a short circuit; instead'it appears to be an inductance. This inductance acts in series with the" equivalent capacitance seen looking towards the load and consequently tends to neutralize this capacitance. The net result of this action is to tend to hold the oscillator frequency substantially at a predetermined value.

Correspondingly, and for reasons that nowwill' be clear, when the frequency goes below the'predetermined value, a corrective capacitance to increase the frequency is applied to the line.

A combination of a resonant cavity with a coaxial line is shown in Figure 3. Here the main line II is broken by a slit 12in the outer conductor. A cavity resonator I3 is fastened on the outside of this slit and serves the same function as the external line section in Figure 1. To accomplish this the cavity must be so dimensioned, in accordance with principles well known to the art, that itresonates in a modesuch that it looks like a short circuit in series with the line at the resonant frequency.

While the embodiments shown in Figures 1 and 2 involve the principle of the invention, there are times when a more powerful stabilizing action is required. For thispurpose the reactance change with frequency of the stabilizing network should be relatively great. This is obtained by having an exceedingly low'L to C ratio in the tuned circult.

It. is inconvenient on straightforwardly constructed tuned circuits having the required L to C'ratio, but over a narrow band its performance can readily be supplemented by means of other types of networks.

Another specific arrangement'of my invention is described with reference to Figure 4. The main transmissionline [2i and I22 is of the coaxial type, with an Outer conductor I2I shown section, and an' inner conductor I22. A branch system is joined to the main coaxial line at ajunction point I24 which isso chosen that an increase in'the apparent inductance of the load as viewed from the junction point would cause the frequency to increase at amaximum rate. At the normal frequency, the impedance-looking into the branch section at junction I24 is infinite.

In orderfor this to'be true; the impedance at junction I25 which is the junction between a lossy section of line I26 and a cavity resonator I21, must be infinite because thelength of line between junctions I24 and I25 is a half wave length. The impedance looking into'the 1055? section of line is primarily resistive, and is in series with the impedance-looking'into the cavity resonator from I25; This cavity resonator impedance is made infinite by adjustment of the tuning screw I28. Thusthe impedance of the branch at junction point I214 is infinite.

At'resonant frequency, the cavity looks like a very high impedance in series with the line. Since the cavity is a half wave from the main transmission linewhen it appears as a nearly open circuit, the stub looks substantially like an open circuit to the transmission line. When, however, the cavity looks like a relatively low'impedance, i. e.-, when off resonant frequency, the low value of impedance is limited by series mis matching resistor 1', which prevents the stub from ever short-circuiting the main transmission line.

When the frequency is slightly raised, the impedance of a cavity coupled in this manner becomes a capacitive reactance, as is'well known in micro-wave theory, andso the impedance in parallelwith the line at 124 becomes capacitive, which is whatis required for the regulatory action described for Figure l to take place.

When the frequencyis far from the cavity resonant frequency, the impedance of the cavity is low, so the net impedance at junctions I24 and I25 is the resistance of'thelossy'line. This prevents short-circuiting of the load at frequencies faraway from resonance, and thus acts to prevent oscillator operation at a frequency of any other oscillation'mode.

Figure 5 is a coaxial line'embodiment of a series connection and is noteworthy for its compactness. The cavity and'resistive material are ar ratio than the impedance between I- and 2.

ranged in a manner electrically similar to the structure of Figure 4 except that the cavity is an odd number of quarter waves from the junction with the main line, which is of the type shown in Figure 2.

In Figure 5 the stub itself however is more complex than the one in Figure 2. The cavity resonator is coupled to the stub at a distance from the series junction I43 equal to an odd number of quarter wave lengths of the frequency to be stabilized. The cavity I44 is coupled to the stub line through a slot I45. The remaining part of the stub line is loaded at its end with some lossy material I46.

The cavity is coupled to the line section in physically the same but electrically a different way than the cavity I3 is coupled to line 1| in Figure 3. This electrical difference arises from the fact that now the cavity is resonating with the electrical lines parallel to the axis of the cavity.

The resonant cavity I3 is magnetically coupled to the transmission line I I The electric coupling cancels out over the whole interior space and accordingly it acts as if it were coupled purely by inductance at the resonant frequency of the tuned circuits shown in Figure 6.

A very high impedance limited only by losses in the circuit appears across the terminals I and 2 of Figure 6. At this same resonant frequency a very high impedance also appears across the terminals 3 and 4. However, it is only across a narrow band that the impedance across the terminals 3 and 4 is similar in form to that across I and 2. This impedance between terminals 3 and 4 differs in two particulars from that across I and 2. In the first place, it appears to be the impedance of a tuned circuit of vastly greater C to L The apparent C to L ratio depends on the proportion of inductance included between 3 and 4. The larger this inductance, the less the C to L ratio. On the other hand, it differs also in the particular that at some higher frequency the impedance between 3 and 4 goes to zero. This frequency is so far removed in the actual designs of cavities used as to be immaterial and the device is used only in the neighborhood of We which is the resonant frequency of the tuned circuit.

In this mode of cavity resonance, the structure of Figure 3 therefore has the property that when d the cavity I3 is in a resonant condition the coaxial line is completely open-circuited opposite. the slot 12.

The result at the junction of I45 is thatthe cavity presents a high resonant impedance in series with the impedance of the rest of the stub which is a resistive impedance. At the frequency to be stabilized, the resonant impedance of the cavity is infinite, so the series resistance of the lossy stub is of no consequence.

The transformation action caused by the odd quarter wave length section of line makes the impedance of the stub at the series junction I43 appear as a short circuit at the frequency to be stabilized. If, however, the load of the main line is changed so that the change in load causes the impedance looking towards the load from junction I43 to become capacitive, the frequency of the oscillator will rise because the series junction I43 is so placed on the main transmission line that this will occur.

This rise in frequency causes the resonant impedance of the cavity to become less than infinite and to become capacitive. It is still high, however, and consequently the impedance corresponding to the increase in frequency causes the impedance at the junction I 45 to become capacitive. At this junction, the series resistance of the lossy section of the stub is negligible. This impedance when transformed by the odd quarter wave section of transmission line appears as a low inductive reactance at the series junction I43. This inductive reactance then compensates for the capacitive reactance of the load as seen from the location I 43. As a consequence, the frequency need change only by the amount necessary for this compensation to occur.

If now the oscillator should attempt to oscillate at a frequency far removed from the desired operating frequency, the impedance of the cavity would become low. This impedance in series with the impedance of the lossy section of the line presents substantially a resistive impedance to the end of the odd quarter wave length section of stub line. The transformation property of this odd quarter wave length section is such that there will appear at the series junction a resistive impedance looking into the stub. This impedance will be in series with the load impedance and consequently the oscillator will still be loaded. This will in effect prevent the oscillator from operating at an undesired mode.

If the lossy section of stub line had not been present, the impedance at series junction I43 under these circumstances are removed from the resonant frequency of the cavity; that is from the desired operating frequency would become an open circuit. This open circuit would effectively disconnect the load from the oscillator and might lead the oscillator to operate at a frequency far removed from the desired frequency. Not all oscillators, of course, have this difliculty, but magnetrons in particular do have a tendency to oscillate at a second frequency if they are unloaded at that frequency. The function of the lossy section will in that case prevent such undesirable oscillation frequencies.

Experimental work with frequency stabilizers of this type indicate that they are also effective in reducing the frequency change caused by variations in voltage or current in the oscillator tube. They find application in radar systems where reflected waves, such as may come from the antenna housing, effect an impedance change in the antenna load.

Various modifications of the principles of my invention will now be evident to those skilled in the art. I therefore prefer not to be bound by the specific disclosures hereinabove set forth, but only by the appended claims.

I claim:

1. In a coaxial cable transmission system for connecting a source of high frequency to a load whose impedance changes, a main coaxial cable having an opening in its outer shield, a branch coaxial cable having an open end and a lossy closed end, the outer shield of the branch being connected to the outer shield of the main coaxial cable at their respective openings, a cavity resonator connected to the outer shield of said branch coaxial cable at a second opening in the outer shield, said connection being a half wave length distance at the frequency to be stabilized from the connection of said branch to said main coa'xial cable, said cavity resonator presenting infinite impedance at the frequency to be stabi- 7 1ized and the lossy end of said branch presentpresenting inductance thereto to drop the frequency.

10. In a coaxial cable transmission system, a coaxial cable, a magnetron oscillator source having two or more modes of oscillations connected to one end of said coaxial cable, a load whose impedance changes connected to the other end of said coaxial cable, said coaxial cable connecting said load to said source of high frequency, said coaxial cable having positions therealong at which the addition of capacitance raises the frequency of said source, the outer shield of said cable being broken at a point where inductance introduced into the transmission line decreases the frequency and a capacitance introduced into the line raises the frequency, a hollow cylindrical tube closed at both ends, one end thereof being connected to the outer shield of said main coaxial cable, said tube extending over and enclosing said break in the main coaxial cable and connecting at its opposite end to the outer shield of said main coaxial cable, the portion of the outer shield of the main coaxial cable enclosed by said tube acting as the inner conductor and said cylinder acting as the outer shield of a coaxial cable, said cylinder being an integral number of halves of the wave length of the frequency to be stabilized.

11. In a coaxial cable transmission system, an oscillator whose frequency is subject to fluctuations in response to changes in load impedance, a load whose impedance changes, a main coaxial cable connecting said oscillator with said load,

10 said cable, when energy is applied thereto from said oscillator, having points therealong one-half wave length apart where an inductive reactance applied to said cable will elevate and a capacitive reactance applied to said cable will lower the frequency of said oscillator, and a stub of a predetermined length with relation to the frequency connection to said coaxial cable for controlling the impedance of said line, said stub connection being at one of said points to introduce frequencysensitive reactance as these values of the load change to maintain a substantially constant frequency of said oscillator in said transmission system.

WILLIAM E. BRADLEY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Disclaimer 2,485,03L-Willz'am E. Bradley, Swarthmore, Pa. HIGH FREQUENCY TRANS- MISSION SYSTEM. Patent dated Oct. 18, 1949. Disclaimer filed Sept. 28, 1951, by the assignee, Phz'lco Corporation.

Hereby disclaims from the scope of claim 6 of the said Letters Patent any apparatus in Which the positions along said coaxial cab-1e at which the addition of capacitance raises the frequency of said source are not spaced substantially one-half Wavelength apart, in Which said coaxial cable stub is not substantially an odd number of quarter Waves long at the frequency of said source and in Which said coaxial cable stub is not connected to said main coaxial cable at a position substantially intermediate adjacent ones-of said first-named positions, said claim being limited hereby to apparatus in Which said positions along said coaxial cable at Which the addition of capacitance raises the frequency of said source are spaced substantially one-half Wavelength apart, in Which said coaxial cable stub is substantially an odd number of quarter Waves long at the frequency of said source and in which said coaxial cable stub is connected to said main coaxial cable at a position substantially intermediate adjacent ones of said first-named positions.

For the same reason hereby disclaims from the scope of claim 11 of the said Letters Patent any apparatus in which the stub connection to the coaxial cable is" not connected in shunt with said coaxial cable and is not of such predetermined length as to provide an eii'ective parallel resonant circuit at the frequency of said oscillator, said claim being limited hereby to apparatus in which the stub connection is in shunt With said coaxial cable and is of such predetermined length as to provide an effective parallel resonant circuit at the frequency of said oscillator.

[Oficz'al Gazette November 6, 1951.] 

